Fabrication and Measurements of GaAs Nanowire Array Solar Cell Devices
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Radial junction p-i-n GaAs nanowire array solar cell (NWASC) devices have been fabricated and optoelectrically measured. Substrates for nanowire (NW) growth were fabricated by electron beam lithography or nanoimprint lithography. The as grown nanowire arrays (NWAs) were processed by deposition of an interlayer dielectric (ILD) and indium tin oxide (ITO) transparent conductive contact. The ILD process has been optimized so that its thickness can be precisely controlled from 500 nm up to 7 μm by varying the viscosity of the dielectric. High bandgap Al0.2Ga0.8As or AlAs layers were grown on the NWs in situ to reduce occurrence of surface recombination centers which can lead to poor optoelectrical properties. Optoelectrical measurements showed that the best solar cell performance in this study gave an open circuit voltage of Voc = 0.25 V, short circuit current density Jsc = 27 Am−2 and fill factor = 0.5 which resulted in a power conversion efficiency (PCE) of η = 0.34%. This is the first demonstration of a photovoltaic device from the NWs grown in the NW group at NTNU. Single NW current voltage measurements revealed low p-doping in the NW cores grown with As2 gas, which could explain the low η. To increase the η in future devices, the various interfaces of the device can be separately measured to find out if they are ohmic or if they are parasitic Schottky contacts. Also, the doping of the NWs should be investigated and optimized to realize a high efficiency.